The present invention relates to gas turbine engines for aircraft and, more particularly, to gas turbine engines each coupled to a corresponding auxiliary engine.
Gas turbine engines as continuous combustion, open Brayton cycle internal combustion engines have come to dominate as the power plants for larger, faster aircraft to essentially the exclusion of reciprocating engines, or internal, intermittent combustion engines, earlier used as power plants for these kinds of aircraft. This is largely because of the greater power-to-weight ratio of gas turbine engines versus internal combustion engines, especially in large horsepower engines, or, more appropriately, large thrust engines in which those large thrusts are provided with a relatively small, and so smaller drag, frontal area engine structures relative to reciprocating engines. Gas turbine engines once having been started, often by an electrical starter typically coupled to the engine high compression compressor and supplied electrical power from an auxiliary power unit, generate large thrusts for propulsion, or large horsepower for engines with an output shaft. They do so by combining large volumes of air with large amounts of fuel, and thereby form a jet of large velocity leading to the capability to provide desired speedy flights.
In addition to providing thrust, such gas turbine engines have coupled to integrated drive generators to operate them to generate electricity for the aircraft and for the engine electronic controls. The amount of electricity needed for these purposes in the past has tended to be relatively modest typically in the range of a few hundred kilowatts of electrical power but, with recently arriving new aircraft, exceeding a megawatt of power. However, there are some aircraft, usually for military uses, that have come to have needs for much larger amounts of electrical power either on a relative basis, the electrical power needed relative to the capability of the gas turbine engine available, or on an absolute basis with power needs significantly exceeding a megawatt. Furthermore, such demands for electrical power in military aircraft often occur at relatively high altitudes and often occur unevenly over relatively long time durations of use, that is, large peaks repeatedly occur in electrical power demand in the course of those long use durations.
Corresponding attempts to obtain the added power from the typical aircraft propulsive system, the gas turbine engine, that are needed to operate the concomitant much larger capacity electrical generators, either on a relative or absolute basis, will subtract significantly from the thrust output of the available turbine or turbines. Making up that thrust loss in these circumstances by operating such available turbine engines so as to increase the thrust output thereof causes the already relatively low fuel use efficiency during flight to decrease significantly, which can severely limit the length of otherwise long duration uses, and also brings those engines closer to becoming operationally unstable.
One alternative to using the gas turbine engine as the sole source of power to operate an electrical power generator is to add in the aircraft a further intermittent combustion internal combustion engine, such as gasoline engines operating on the any of the Diesel, Miller, Otto or Wankel cycles. Such engines can operate with a fuel efficiency on the order of seventy percent (70%) better than that of a continuous combustion (Brayton cycle) internal combustion gas turbine engine. At high altitudes, internal combustion engines of all kinds face the problem of limited power output because of the relatively small air pressures there limiting the chemical reactions of oxygen with hydrogen and oxygen with carbon in the burning of the engine fuel in the engine combustion chamber or chambers. This can be solved for gas turbine engines by providing therein very large air flows through use, typically, of axial flow compressors usually in two stages with both a low compression compressor followed along the fluid flow path through the engine by a high compression compressor. This arrangement provides at least enough compressed air to the subsequent combustor to sustain the desired combustion process therein and a mass of airflow sufficient to combine with enough fuel to provide the energy needed to overcome the aircraft drag at the speed and altitude intended for operation.
However, such compressors can provide considerably more compressed air than the minimum needed for this purpose thereby allowing some of this compressed air to be delivered through an air transport duct to the air intake of an intermittent combustion internal combustion engine so that, in effect, the compressors of the gas turbine engine serve as a very capable supercharger for that intermittent combustion engine. Thus, this intermittent combustion engine can be operated at the same relatively high altitudes at which the gas turbine engine propelling the aircraft operates while this turbine engine is also supplying compressed air to that intermittent combustion engine. There, depending on the values selected for the peak air intake pressure and engine compression ratio, the intermittent combustion engine can be used as a power source for an electrical power generator that can generate much greater amounts of electrical power than can one powered by a gas turbine engine.
Although such large amounts of electrical power are needed in operating various devices in the aircraft, they are usually fully or substantially needed only during certain portions of a flight, and are much less needed during other flight portions thereby idling the internal combustion engine during those portions. Such an intermittent combustion engine can be put to a further use in an aircraft of the kind that has the gas turbine engine used therein positioned within walls thereabout of a duct with the inlet side of that duct curved to follow a sinuous path to hide the front of that engine from impinging electromagnetic radiation at various wavelengths such as in a stealth type military aircraft (several kinds of which are unmanned aircraft). Typically, much of the inlet duct portion has a cross sectional area more closely approximating an elliptical shape rather than round so that the desired curves in the duct along its extent can be completed over a shorter extent distance, and then the duct cross section changes to being more round at the gas turbine engine location to accommodate that engine. The amount, or sharpness, of the curvature of the inlet portion of the duct, reflected in the curvature of the curve of cross sectional symmetry of that duct along its extent, resulting from the need to achieve the desired hiding of the front of the gas turbine engine depends on the space available for the duct in the aircraft and the size of that engine. That is, the length, L, of the duct curve of cross sectional symmetry from the duct opening to the atmosphere, on one end thereof, to the front of the gas turbine engine on the other end, and the diameter, D, of the front of that engine provide in their ratio L/D a parameter indicative of the curvature of the inlet portion of the duct, and so the compactness of this convoluted duct part and how extreme must be the resulting directional turning of airflows therethrough.
Relatively slow aircraft speeds at which there is little ram effect forcing air into the inlet duct portion such as occur after takeoff of the aircraft from a runway, followed by relatively sharp climb angles with respect to the aircraft flight direction, and the like, lead to separation or separations of the air flows in this inlet duct from the walls of that duct at locations therein just past the relatively sharp curves occurring in this duct in the direction of extent thereof. Regions of such flow separations from the duct walls extending to the gas turbine engine such as a turbofan engine can lead to stalling of the engine fan or cause individual fan blades to flutter and then structurally fail before the aircraft reaches speeds sufficient for the air entering the inlet duct portion to reach such ram pressures as to prevent these separations. Different ratios L/D for the inlet duct portion in aircraft having engines positioned in a duct will lead to different duct path turning angles and turning radii occurring therealong especially at those duct locations just before and past relatively sharp curves in the duct path. Air flow separations inwardly just past these curves will be less likely with less duct curvature along the duct path but reducing curvature may also negatively affect the positioning of the gas turbine engine in the aircraft. Thus, such duct curvature may nevertheless be required along with any of the likely air flow separations at these locations having to be tolerated.
During takeoff from a runway of an aircraft containing such a gas turbine engine and intermittent combustion engine and the following climb to gain altitude, this intermittent combustion engine is unlikely to be needed to provide torque to an electrical generator for the purpose of its generating large amounts of electrical power while still near that runway. In this portion of the flight, relatively slow aircraft speeds occur leading to the result that air is not forced with substantial pressure (ram pressure) into the inlet of the inlet portion of the duct in which the gas turbine engine is positioned. In this circumstance, often compounded by the relatively sharp climb angles with respect to the aircraft flight direction used after takeoff to gain altitude, the separations of the air flows in this inlet duct from the walls of that duct at locations therein just past the relatively sharp curves provided in this duct can occur as indicated above.
Thus, the intermittent combustion engine is available in at least this part of the flight to aid in preventing such separations without having to be sized sufficiently to provide concurrently both such aid and torque to electrical generators. This engine can do so by establishing a reduced pressure at the potential separation locations during this part of the flight by drawing air through openings located there to help force the duct flows to remain flowing along these portions of the duct walls. Other uses of the intermittent combustion engine to more fully employ it in the aircraft and to provide some reserve against its in-flight failure are also desired.